This invention relates to a process for the continuous preparation of aromatic diazonium fluoride salts.
Diazotization of aromatic primary amines to prepare aromatic diazonium fluorides is of considerable importance in that such diazonium compounds are commercially valuable intermediates which can be converted to dyes (e.g. amino azobenzene) and aromatic fluorides which are useful as intermediates in preparing various pesticides, pharmaceuticals and other products.
Various processes have been used in the past to prepare aromatic-fluoro compounds. None of these prior art processes has proven entirely satisfactory. For example, Japanese patent publication No. 81330-74 describes a process for the preparation of aromatic fluoro compounds on a laboratory, as opposed to an industrial, scale. Diazotization of a substituted or unsubstituted amino compound and the thermal decomposition of the diazonium salt produced by diazotization are carried out in one step at the thermal decomposition temperature of the diazonium salt. A solution of a compound that gives nitrous acid first is dissolved in hydrofluoric acid and then allowed to act on an aromatic amino compound. No methods are suggested for removing the heat produced from this reaction. The examples set forth in the patent illustrate batchwise procedures.
German Pat. No. 600,706 relates to a batch-wise process for the production of fluoro aromatic compounds. The diazotization and decomposition are carried out in the presence of excess anhydrous, or practically anhydrous, hydrogen fluoride. An aromatic amine is dissolved in hydrogen fluoride, and a diazotization agent such as dry sodium nitrite slowly is added over the course of one hour. During this process the temperature is held to about 5.degree. C. This process also is discussed by Ferm et al. in J.A.C.S. 72:4809-4810 (1950). Experimenting on a small-scale, Ferm et al. confirmed the utility of the above-process for a number of fluoroaromatics in batch-wise procedures. None of these processes is amenable to the continuous industrial production of fluoro-aromatics in high yields.
Various processes for the continuous production of fluoro-aromatics also are known. For example, Hupfer, U.S. Pat. No. 3,117,954, describes a crossflow cascaded reactor for the continuous aqueous solution of sodium nitrite. Agitation is limited to effect sedimentation.
Hamilton, et al, U.S. Pat. No. 4,246,171, discloses a continuous diazotization process in which the rate of addition of the inorganic nitrite is controlled by a polarovoltric method. An aqueous solution of an amine in an acid and a solution of an inorganic nitrite are added continuously and regularly to a reactor. The addition of the solution of inorganic nitrite is automatically regulated to ensure that a preselected concentration of unreacted nitrous acid is maintained in the reactor throughout the entire reaction period. Hamilton et al. recognize that amines which give a fast diazotization reaction rapidly produce a very large amount of heat. Hamilton et al. also recognize that unwanted by-products will form if the temperature rises too high. The use of flaked or crushed ice is suggested to control this rise in temperature.
Aqueous systems typically result in lower yields, about 50% after decomposition. Moreover, the use of aqueous systems in conjunction with HF systems results in additional problems, such as increased corrosion of the reaction vessels.
Prior art diazotization processes have not proven completely satisfactory. They typically are not amenable to a continuous process or are complex, inefficient, expensive and/or prone to result in an unacceptably high level of unwanted by-products. Most of these deficiencies are caused by the problem of the tremendously exothermic nitrite/HF and diazotization reactions.
Accordingly, there is a substantial need in the art for a continuous diazotization process which would overcome the disadvantages of heretofore known processes.